A designer incorporates a control system to control position of a device, for example a flow control valve that manages air intake on an internal combustion engine. A typical control system comprises the device, e.g. the flow control valve, with an electromechanical actuator and a position sensor, a controller, and a commanded input from an external input signal. The controller controls applied force to the electromagnetic actuator, based upon the commanded input and feedback from the position sensor, to control position of the device. Performance parameters for such systems typically include a measure of the ability of the device to respond to the input command in terms of response time and settling, and an ability of the device to maintain a stable position.
When the device is the flow control valve, the applied force necessary to achieve and maintain a specific position of the flow control valve is variable depending upon the position of the valve. When the flow control valve is the air management valve, the applied force typically comprises the force necessary to overcome load force on the electromagnetic actuator, including for example, bearing friction and return-spring force, and an air load force from flow of air around the valve. When the air management valve comprises an electronic throttle control device on an internal combustion engine, load forces typically include bearing friction, return-spring force, and an air load force resulting from engine pumping. The applied force necessary to achieve and maintain the specific position of the flow control valve may also be affected by its direction of rotation. Each of the aforementioned load forces is further affected by component manufacturing tolerances and interferences, ambient conditions, and component wear and cleanliness.
The control system is typically executed as algorithms and calibrations in the controller. Control system designers employ traditional control strategies, including proportional, integral, and derivative terms, to achieve acceptable control over position of the device. The designers employ a steady-state position error in a simple proportional (P) or proportional-plus-derivative (PD) control algorithm to counteract load forces on the flow control valve. When the load force is a predictable function of flow control valve position, a feedforward term may be added to the P or PD control algorithm to provide the applied force, in terms of an actuator energization signal, necessary to counteract the load force. A predictable load force caused by friction is compensated by a calibrated step change in the actuator energization signal and is dependent upon the direction of actuator movement. When the load force varies as a predictable function of flow control valve position, the calibrated step change can vary accordingly. However, the predictability of the applied force required to move the actuator to a desired position is generally limited, due to unpredictable changes in the load force. Changes in the load force include variations in ambient conditions, variations in component design, manufacturing, application, and customer usage, variations in system operating conditions, and others. The aforementioned variations in the load force lead to inaccuracies in a control system that employs a fixed-calibration feedforward term or step change.
Designers may add an integrator with an integral term (I) to the control algorithm to compensate for control system inaccuracies that lead to limited ability to predict the applied force necessary to achieve a specific actuator position. The integrator output changes at a rate that is proportional to the position error, i.e. a difference between monitored position and commanded input to the actuator. The integrator accumulates an offset to the actuator energization signal until the position error is zero. The accumulated integrator output signal required to compensate for changes in load force typically varies with actuator position and direction of movement, thus requiring the integrator to adjust output accordingly when a change in actuator position is commanded.
The integrator may be designed to rapidly adjust to a new steady-state value to improve response of the flow control valve. Rapid adjustment causes the integrator term to quickly accumulate (wind-up) an erroneous value when the position error term is temporarily large. This typically occurs during a sudden, large change in the external input signal. A control system for a flow control valve employing a rapidly adjusting integrator term may be underdamped, leading to unwanted oscillations in the position of the flow control valve actuator, and corresponding system instability.
The integrator may instead be designed to slowly adjust to a new steady-state value, for more accurate response of the flow control valve. Slow adjustment of the integrator term causes the integrator term to adjust slowly to changes in the external input signal, even when the position error term is large. A control system for a flow control valve employing a slowly adjusting integrator term may be overdamped, leading to slow response of the flow control valve to a new position. This may lead to operator dissatisfaction.
Therefore, what is needed is a control system and method for position control of a flow control valve with an electromechanical actuator, such as an air management valve. The preferred control system and method responds rapidly and accurately to the external input signal and adaptively adjusts to changes in the various applied forces on the actuator, including forces applied by the components, external forces, and forces affected by direction of movement of the actuator.